Semiconductor device having an aluminum nitride film

ABSTRACT

A semiconductor device of having an insulating film comprising aluminum nitride and oxygen provided over a rear surface of a substrate. A transistor is provided over the insulating film. The transistor has at least a channel formation region comprising crystalline silicon, a gate insulating film adjacent to the channel formation region, and a gate electrode adjacent to the channel formation region with the gate insulating film interposed therebetween.

This application is a Divisional of application Ser. No. 09/255,777,filed Feb. 23, 1999, now U.S. Pat. No. 6,964,890, which is itself aDivisional of application Ser. No. 08/757,616, filed Nov. 29, 1996, nowU.S. Pat. No. 5,946,561, which is itself a Continuation of applicationSer. No. 08/085,931, filed Jul. 6, 1993, now abandoned, which is itselfa Continuation-in-part of application Ser. No. 07/853,690, filed Mar.17, 1992, now U.S. Pat. No. 5,313,076.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a low-temperature process forfabricating an insulated gate semiconductor device at a temperature aslow as 450° C. or even lower, and to a process for fabricating, at goodyield, an integrated circuit (IC) comprising said devices at a highdegree of integration. The present invention also relates to asemiconductor device having fabricated by the above process. It furtherrelates to a highly reliable semiconductor device. The semiconductordevices according to the present invention are suited for use in, forexample, active matrix-driven liquid crystal displays, driver circuitsof image sensors, etc., as well as thin film transistors for SOIintegrated circuits and for conventional semiconductor integratedcircuits (e.g., microprocessors, microcontrollers, microcomputers,semiconductor memories, etc.).

2. Prior Art

Recently, much effort is paid on the study of fabricating insulated gatesemiconductor devices on insulator substrates (MOSFETs). Such devicescomprising semiconductor integrated circuits on insulator substrates areadvantageous for driving circuits at high speed. In contrast to theconventional semiconductor ICs whose speed is limited by the presence ofa stray capacitance attributed principally to the capacitance betweenthe connection and the substrate, the new type of semiconductorintegrated circuits do not suffer such stray capacitance. The aboveMOSFET having a thin film active layer on an insulator substrate isdenoted as a thin film transistor (TFT). The TFT can be found also inthe conventional semiconductor ICs as, for example, a load transistorfor SRAMs.

More recently, there is a demand for fabricating semiconductor ICs on alight-transmitting substrate, for example, as driver circuits in opticaldevices such as liquid crystal displays and image sensors. TFTs are alsouseful in such application fields. The circuits for use therein should,however, be formed over a large area. The process is therefore requiredto be conducted at a ever lower temperature. Furthermore, for example,when there is a need of connecting a semiconductor IC to the terminalsof a device having a plurality of terminals on an insulator substrate,it is proposed to form monolithically the entire semiconductor IC or toform at least the initial stages thereof monolithically on the sameinsulator substrate.

Conventionally, TFTs have been fabricated by annealing an amorphous, asemi-amorphous, or a microcrystalline semiconductor film in thetemperature range of from 450 to 1,200° C. to obtain a crystalline filmhaving an improved crystallinity and having a sufficiently highmobility. TFTs include amorphous TFTs using an amorphous material as thesemiconductor film, however, such TFTs are not useful as they arebecause they yield a mobility as low as 5 cm²/Vs or even lower, and ingeneral, the mobility falls to a value of about 1 cm²/Vs or lower. Theuse of amorphous TFTs as they are is confined to a narrow range ofapplication because of its low operation speed and its limitedapplicability to N-channel type TFTs. Accordingly, these TFTs wereannealed in the aforementioned temperature range to attain a mobility of5 cm²/Vs or higher. Only after annealing, these TFTs can provideP-channel TFTs (PTFTs).

A thermal process as described in the foregoing has, however, strictlimitations on the material to be used as the substrate. In a so-calledhigh temperature process which comprises a step of heating to atemperature in the range of from 900 to 1,200° C. at maximum, athermally oxidized film of superior quality can be used as the gatedielectric. Thus, expensive substrates such as those made of quartz andsapphire and spinel were the only candidates applicable to such hightemperature processes. Moreover, large area substrates were rarelyobtained with such expensive materials.

In contrast to the case of a high-temperature process, variety ofsubstrate materials can be selected for use in a low temperature processwhich is conducted at temperatures which do not exceed the range of from450 to 750° C. However, a low temperature process requires annealing fora long time, and the substrates resulting therefrom suffer strain andshrinking due to the heat effect.

Furthermore, it is extremely difficult in a MISFET, i.e., an insulatedgate semiconductor device having formed on an insulated surfaceestablished by incorporating a thick insulator film between asemiconductor substrate and the device to isolate the device from thesemiconductor substrate, to obtain an element having favorablecrystallinity as in the case using a single crystal semiconductor.Accordingly, a non-single crystalline semiconductor, i.e., a crystallinesemiconductor other than a single crystal semiconductor, had been usedgenerally in MISFETs.

The non-single crystalline semiconductors comprise defects at highdensity, and are usually neutralized previously with an element such ashydrogen to use them in a practically defect-free state. Theneutralization process can be carried out by, for example,hydrogenation. The bond between hydrogen and the semiconductor elementsuch as silicon is generally weak, and would easily undergo breakage tocause decomposition of the resulting compound on applying a thermalenergy corresponding to a mere several tens of degrees Centigrade.Accordingly, when electric voltage or current is applied for a longduration of time, hydrogen readily undergoes desorption due to the localheat up of the semiconductor. This phenomena remarkably causesdegradation of the semiconductor.

The present invention has been achieved in the light of theaforementioned circumstances. An object of the present invention is,therefore, to provide a process which can be conducted at a temperaturenot higher than 450° C., which suffer no limitations on the substratematerial, and which has no problems of strain and shrinkage. Anotherobject of the present invention is to provide a semiconductor devicehaving such a structure that the heat generated during its usage can berapidly released, and also to a process for fabricating the device.

SUMMARY OF THE INVENTION

A first embodiment of the present invention provides a thin filmsemiconductor device comprising a substrate having provided thereon afilm comprising aluminum nitride as the principal component, havingdirectly or indirectly formed thereon a semiconductor film comprisingsilicon as the principal component, and having further establisheddirectly or indirectly thereon a wiring made of a material such as ametal and a semiconductor.

The present invention also provides a process for fabricating a thinfilm semiconductor device having the above structure. Accordingly, asecond embodiment of the present invention provides a process whichcomprises forming a film containing aluminum nitride as the principalcomponent, forming thereon either directly or indirectly a semiconductorfilm comprising silicon as the principal component, and furtherestablishing thereon either directly or indirectly a wiring made of amaterial such as a metal and a semiconductor.

Aluminum nitride is a superior conductor of heat and is suited forapplications in which light transmitting properties are required,because it has an optical gap of 6.2 eV and is thereby transparent tovisible light and near ultraviolet light. The aluminum nitride film isformed by deposition processes such as sputtering, reactive sputtering,and MOCVD (metal-organic chemical vapor deposition). In obtaining analuminum nitride film by a reactive sputtering process, the process ispreferably conducted under nitrogen gas atmosphere using an aluminumtarget. For achieving sufficient heat emission with the aluminum nitridefilm in accordance with the object of the present invention, thealuminum nitride film is preferably deposited at a thickness of from 100to 5,000 Å. An aluminum nitride film 5,000 Å or more in thickness wasnot practically feasible because the deposited film could be easilypeeled off.

The thus obtained aluminum nitride film exerts a blocking effect againstmobile ions such as sodium. Accordingly, the film protects thesemiconductor device against the intrusion of such mobile ions.

The aluminum nitride film need not contain nitrogen and aluminum at astoichiometric ratio so long as the thermal conductivity of the film isnot impaired. Typically, a preferred aluminum to nitrogen ratio(aluminum/nitrogen) is in the range of 0.9 to 1.4, and the thermalconductivity of the film is preferably 0.6 W/cm·K or higher. This valuecan be contrasted to 2 W/cm·K for single crystal aluminum nitride.

The tension of the film may be controlled optimally by changing thecompositional ratio of nitrogen and aluminum. Otherwise, a trace amountof boron, silicon, carbon, oxygen, etc., may be incorporated tooptimally control the strain. The film containing aluminum nitride asthe principal component may be either crystalline or amorphous.

In general, a high thermal conductivity can be achieved by incorporatinga diamond material such as a thin film of polycrystalline diamond, ahard carbon film, or a diamond-like carbon film. When a small area asthe one in the device according to the present invention is considered,however, a satisfactory effect cannot be obtained because a tightadhesion cannot be obtained between a diamond material and a siliconoxide material. A silicon nitride film which is frequently used in asemiconductor process as a blocking layer and a passivation layer is notsuited in that the thermal conductivity thereof is low. Thecharacteristics of the well known materials for thin films wereevaluated, and the results are summarized below for comparison.

AlN¹⁾ DLC²⁾ SnO₂ ³⁾ SiN_(x) ⁴⁾ Adhesibility⁵⁾ ◯ Δ ◯ ◯ Light ◯ Δ ◯ ◯transmittance Mechanical ◯ ◯ Δ ◯ strength Thermal ◯ ◯ ◯ Δ or Xconductivity Heat ◯ Δ Δ ◯ resistance Sodium blocking ◯ Δ Δ ◯ effect Note¹⁾AlN: Aluminum nitride, ²⁾DLC: Diamond-like carbon, ³⁾SnO₂: Tin oxide,and ⁴⁾SiN_(x): Silicon nitride. ⁵⁾“Adhesibility” signifies adhesibilityto silicon oxide.

The symbols ◯, Δ, and x in the evaluation represent “good,” “fair,” and“poor,” respectively.

In the device according to the present invention, the heat havinggenerated from the metallic or semiconductor wiring (e.g., gate wiring,etc.) is transferred to the underlying semiconductor films (e.g., activelayers, etc.), and the semiconductor films themselves generate heat bythe electric current applied thereto. Accordingly, the semiconductorfilms are heated to a higher temperature, but the heat is rapidlytransferred to an aluminum nitride film provided under the semiconductorfilm to prevent heat accumulation from occurring on the semiconductorfilm. In this manner, the temperature of the wiring and thesemiconductor film can be suppressed to avoid hydrogen desorption.

It is not preferred in the present invention to deposit thesemiconductor film directly on the aluminum nitride film. If thesemiconductor film were to be deposited directly on an aluminum nitride,not only the adhesion results insufficient, but also an unfavorableinfluence is cast on the electric properties of the semiconductor film.Accordingly, it is preferable to provide, between the semiconductor filmand the aluminum nitride film, a material effective for stressrelaxation and yet having favorable electric and chemical properties.

Alternatively, a silicon nitride film may be formed with an aluminumnitride film thereon, and a silicon oxide film may be further formedthereon. In the device according to the present invention, the gatecontact may be made from single elements such as silicon (inclusive ofan impurity-doped one having an improved conductivity), aluminum,tantalum, chromium, tungsten, and molybdenum, or from an alloy or amultilayered film thereof. Furthermore, the surface thereof may beoxidized as described in the Examples referred hereinafter.

Aluminum nitride may be used positively as an etching stopper, becauseit would not be etched by any etching method commonly used for etchingsilicon oxide, silicon, aluminum, etc., in an ordinary fabricationprocess for semiconductor devices.

The process according to the present invention is also characterized bythat the crystallinity of the semiconductor film is ameliorated not by aconventional process in thermal equilibrium, but by the irradiation ofan intense light such as a pulsed laser beam or an intense lightequivalent thereto. By employing this method, it can be seen that themaximum temperature of the process depends on the temperature of thestep other than the annealing of the semiconductor film, that is, on thesteps such as the hydrogenation annealing and the annealing of gatedielectric. Accordingly, the substrate for use in the device accordingto the present invention can be selected from a wider range ofmaterials. More specifically, a soda-lime glass or an alkali-free glass(e.g., #7059 glass from Corning Incorporated), which were regardedconventionally unapplicable to substrates for operating TFTs thereon dueto the low softening point thereof, can be used for driving TFTs afterapplying a pertinent treatment to the glass.

The process according to the present invention comprises forming asemiconductor film on an insulator substrate; forming a film capable oftransmitting a laser beam or an intense light equivalent to the laserbeam on said semiconductor film; irradiating a pulsed laser beam or anintense light equivalent to the laser beam to said layered film tothereby improve the crystallinity of the semiconductor film; removingsaid film capable of transmitting the laser beam or the intense light toexpose a surface of the semiconductor film; forming a gate insulatingfilm on said semiconductor film; forming a wiring or a gate contact onsaid gate insulating film; introducing impurities into saidsemiconductor film in a self-aligned manner with the wiring or the gatecontact as a mask by processes such as ion irradiation and ionimplantation and ion doping; and irradiating a pulsed laser beam or anintense light equivalent thereto to said semiconductor film with thewiring or the gate contact as a mask after the introducing step tothereby recover the crystallinity of the semiconductor film which wasonce destroyed in the step of introducing the impurity elements. Thelast two steps may be replaced by laser doping process disclosed in theapplication of the present inventors (see, for example, Japanese PatentApplication No. Hei-4-100479). In the present invention, metallicmaterials having low resistivity, such as aluminum, are preferred foruse as the materials for gate contact and connection. The pulsed laserbeam for use in the present invention is generated preferably fromultraviolet-light emitting lasers such as excimer lasers using KrF, ArF,XeCl, and XeF gases. Preferably, an insulator film of a materialselected from silicon nitride, aluminum oxide, and aluminum nitride, ora layered film composed of the same with silicon oxide film is providedbetween said insulator substrate and said semiconductor film. Thesilicon oxide film is provided at a thickness of from 300 to 3,000 Å,and more preferably, at a thickness of from 500 to 1,500 Å. Theinsulator film of a material selected from silicon nitride, aluminumoxide, and aluminum nitride is provided at a thickness of from 300 to3,000 Å, and preferably, at a thickness of from 1,000 to 2,000 Å.Otherwise, a halogen infrared light-emitting lamp may be used forirradiating an intense light. An intense light (or a pulsed light)equivalent to a laser beam signifies an optical energy or a combinationthereof with an auxiliary thermal energy, which is applied for asufficiently short period of time, in general, for a duration of within5 minutes, to the semiconductor film for recovering the crystallinitythereof.

The present invention is characterized by that after removing thepreviously established protective layer used for irradiation of a laserbeam or an intense light equivalent thereto to the active layer torecover the crystallinity of the active layer, a film other than theprotective layer may be used as the gate insulating film. This stepconsiderably improves the characteristics of the resulting TFT. Thereason for the improvement in the characteristics of a TFT is believedas follows. In the crystallization from an amorphous state, aconsiderable amount of non-stoichiometric compounds are often found todevelop at the interface, and particularly, silicon-rich silicon oxidetend to form at the vicinity of the interface. Those non-stoichiometriccompounds, however, function insufficiently either as insulators orsemiconductors. It can be seen accordingly that the presence ofnon-stoichiometric silicon oxide hinders the achievement of preferredcharacteristics since it is well established that the interface plays animportant role in an insulated gate element.

If a laser beam or an intense light equivalent thereto is irradiateddirectly onto the film without using any protective film, however, anirregular surface is developed thereon. Such an uneven surface as aconsequence provides an element having poor characteristics. The step ofremoving the once provided protective layer corresponds to the removalof the aforementioned non-stoichiometric silicon oxide to give puresilicon with favorable crystallinity. In particular, it is found thatfavorable results can be obtained by removing the protective layer bywet etching using hydrofluoric acid and the like. A dry etching processcauses damage to the silicon film, but wet etching provides an extremelystable surface by terminating the dangling bonds with fluorine andhydrogen before double bonds are formed among the silicon atoms.

In the present invention, the depth of the region which form uponannealing with a laser beam or an intense light equivalent thereto canbe set and controlled freely as desired according to the invention ofthe present inventors as disclosed in Japanese Patent Application No.Hei-3-50793. In this manner, a structure comprising a double-layeredactive layer can be obtained to reduce the leak current between thesource and the drain.

The annealing process using a laser beam or an infrared (IR) light froman IR lamp according to the present invention is preferably conductedwhile heating additionally the substrate to a temperature of from 100 to500° C., and representatively, to 300 to 400° C. In this manner, a filmwith improved homogeneity can be obtained.

A first example for the application according to the present inventionprovides a peripheral circuit for an active-matrix (AM) driven liquidcrystal display (LCD) device using an amorphous silicon (a-Si) TFT. Thea-Si TFT-AMLCD can be obtained by establishing an a-Si TFT generally ata temperature range of 400° C. or lower on a substrate made from analkali-free glass such as Corning #7059 glass (produced by, CorningIncorporated). An a-Si TFT has a high OFF resistance and is thereby bestsuited for a switching element, however, as mentioned earlier, it cannotprovide a CMOS and it suffers a low operation speed. Accordingly, theperipheral driver circuit is generally established with a single crystalIC and the terminals of the matrix are connected to the terminals of theIC by methods such as tape-automated bonding (TAB). However, this typeof mounting confronts more difficulty in reducing size of the pixels,and thereby the cost for mounting increases as to account for a largerpercentage of the module cost.

It had been difficult by the conventional process to establish theperipheral circuit on the same substrate for the matrix due to thermalconstraints. In the present invention, however, a TFT having a largermobility can be established at a temperature equivalent to that at whicha conventional a-Si TFT has been formed.

A second example for the application of the present invention comprisesforming a TFT on a material such as soda-lime glass, i.e., a glassfurther reduced in cost as compared with an alkali-free glass. In thiscase, preferably, an insulator coating is first applied to the glass toavoid direct contact of the TFT with the soda-lime glass, because themobile ions such as sodium ions intrude from the glass into the TFT. Theinsulator coating may be such containing silicon nitride, aluminumoxide, or aluminum nitride as the principal component. Then, a baseinsulator film made from silicon oxide and the like is formed on theresulting insulator coating, and the process according to the presentinvention is applied to establish a TFT. Furthermore, failure of thedevice can be avoided by preferentially using PTFTs over NTFTs as thematrix TFTs. When mobile ions intrude into an NTFT from the substrate, achannel is always formed to realize an ON state on the NTFT. However, aPTFT would not suffer formation of a channel in such a case.

A third example for the application of the present invention comprises aperipheral circuit of a liquid crystal display (LCD) of a directmultiplexing drive type, i.e., a static simple-matrix driven LCD. Aferroelectric liquid crystal (FLC), for instance, has a memory function,and it thereby provides a display of high contrast even when it issimple-matrix driven. Conventionally, however, the peripheral circuittherefor has been established in the same manner as in the a-Si TFTAMLCDs by connecting the ICs by a TAB process and the like. Similarly,the peripheral circuit for a static operation LCD which takes advantageof the phase transition from a cholesteric phase to a nematic phase hasbeen conventionally established by TAB connection. A static drive LCDwhich comprises a combination of a nematic liquid crystal and aferroelectric polymer is proposed in JP-A-61-1152 (the term “JP-A-” asreferred herein signifies “an unexamined published Japanese patentapplication), however, this also comprises a TAB-connected peripheralcircuit.

All of the LCDs enumerated above are of direct multiplexing drive andthey therefore provide a large area display with high precision using alow cost substrate. A fine display can be obtained by reducing the pitchbetween the terminals, but only at the expense of making the IC mountingdifficult. It can be seen, accordingly, that the present inventionprovides monolithically a peripheral circuit using a low cost substrateand yet free from concerns on the problem of heat.

A fourth example for the application of the present invention provides aso-called three-dimensional IC which comprises forming TFTs onsemiconductor ICs having established thereon metallic connections. Stillother and a variety of applications are available taking advantage ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, (A)-(E), are cross-sectional views of a TFT, for illustratingsteps successively performed to fabricate the TFT according to theinvention;

FIG. 2, (A)-(E), are cross-sectional views of another TFT, forillustrating steps successively performed to fabricate the TFT accordingto the invention;

FIG. 3, (A)-(D), are cross-sectional views of a further TFT, forillustrating steps successively performed to fabricate the TFT accordingto the invention;

FIG. 4, (A)-(C), are diagrams illustrating the principle of operation ofan LCD according to the invention;

FIG. 5 is a cross-sectional view illustrating the cell structure of anLCD according to the invention;

FIGS. 6, (A) and (B), are graphs showing the characteristics of a TFTaccording to the invention;

FIG. 7, (A)-(D), are cross-sectional views illustrating a method offabricating a TFT according to the invention;

FIG. 8, (A)-(E), are cross-sectional views illustrating another methodof fabricating a TFT according to the invention;

FIG. 9, (A)-(E), are cross-sectional views illustrating a further methodof fabricating a TFT according to the invention;

FIG. 10, (A)-(E), are cross-sectional views illustrating a still othermethod of fabricating a TFT according to the invention;

FIG. 11 is a front elevation partially in circuit diagram of anactive-matrix circuit and a peripheral circuit according to theinvention;

FIG. 12, (A)-(E), are cross-sectional views illustrating yet anothermethod of fabricating a TFT according to the invention;

FIG. 13, (A)-(D), are cross-sectional views illustrating yet a furthermethod of fabricating a TFT according to the invention;

FIG. 14, (A)-(D), are cross-sectional views illustrating an additionalmethod of fabricating a TFT according to the invention;

FIG. 15, (A)-(D), are cross-sectional views illustrating a stilladditional method of fabricating a TFT according to the invention;

FIG. 16, (A)-(E), are cross-sectional views illustrating a still furthermethod of fabricating a TFT according to the invention; and

FIG. 17, (A)-(E), are cross-sectional views illustrating yet anothermethod of fabricating a TFT according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is illustrated in greater detail referring to anon-limiting example below. It should be understood, however, that thepresent invention is not to be construed as being limited thereto.

EXAMPLE 1

A peripheral circuit for an active matrix (AM)-driven liquid crystaldevice (LCD) using a-Si TFT was established according to the presentinvention. As mentioned in the foregoing, conventional AMLCDs based ona-Si TFTs had been fabricated by TAB connection because it was notpossible to form the peripheral circuit monolithically with the matrix.However, a TAB process is costly due to the high cost of connectionwhich is necessary in addition to the cost of the ICs. The total cost ofthe ICs and the connection amounted to account for 20% or more of thecost for the entire panel module. A low cost panel module was realizedaccording to the present invention, by establishing the matrix and theperipheral circuit monolithically on a single glass substrate.

First, a silicon oxide film 102 as a base oxide film was formed to athickness of from 100 to 300 nm on a Corning #7059 glass substrate 101(either 300 mm×300 mm or 100 mm×100 mm in area) by sputtering in anoxygen atmosphere or by decomposing TEOS and depositing silicon oxide byplasma CVD, which was followed by annealing in the temperature range offrom 450 to 650° C.

Then, an amorphous silicon film 103 was deposited by plasma CVD or LPCVDat a thickness of from 30 to 150 nm, preferably from 50 to 100 nm, and asilicon oxide or a silicon nitride film was deposited thereon as aprotective layer 104 at a thickness of from 20 to 100 nm, preferably, ata thickness of from 50 to 70 nm. A KrF excimer laser pulse beingoperated at a wavelength of 248 nm and at a pulse width 20 nsec wasirradiated to the amorphous silicon film 103 through the silicon oxideor silicon nitride film to improve the crystallinity of the silicon film103. This step is shown in FIG. 1(A). Laser beam was applied at anenergy density of from 200 to 400 mJ/cm², and preferably, at a densityof from 250 to 300 mJ/cm². The crystallinity of the silicon film thusdeposited was studied by Raman scattering spectroscopy to identify arelatively broad peak at about 515 cm⁻¹, i.e., a peak different from apeak assigned to single crystal silicon which should appear at 521 cm⁻¹.A further uniform crystal can be obtained by applying an auxiliary heatthereto by heating at 100 to 500° C. during the laser irradiation. Theresulting crystal was then annealed in hydrogen at 350° C. for 2 hours.

Subsequently, the protective layer 104 was removed to expose the siliconlayer 103, and the exposed surface was patterned into an island-likeshape to establish an NTFT region 105 and a PTFT region 106.Furthermore, a film having obtained by sputtering in an oxygenatmosphere or by decomposing TEOS and depositing using plasma CVDprocess was further annealed at a temperature range of from 450 to 650°C. to form a gate oxide film 107. Sufficient care should be taken intreating a large area substrate by the latter process, i.e., the plasmaCVD process, because strain and shrinkage may form on the substrateduring the process depending on the heating temperature. If such astrain and shrinkage should generate on the substrate, difficultiesshould be found in the mask alignment process which is to be conductedin the later fabrication step. In the sputtering process, on the otherhand, the substrate can be maintained at a temperature of 150° C. orlower. It is preferred, however, that an annealing is conducted at about450° C. in hydrogen to reduce the dangling bonds and the like inside thefilm to thereby prevent the gate oxide film from being influenced by thefixed charges.

An aluminum film was further deposited thereafter to a thickness of from200 nm to 5 μm by electron beam vapor deposition process, and wasthereafter patterned to obtain gate contact 108 and 109 as illustratedin FIG. 1(B). A gate contact 110 of the TFT (reverse stagger type) ofthe active matrix circuit was formed simultaneously with the formationof the gate contact 108 and 109.

FIG. 1(C) illustrates the manner of forming anodic oxide layers 111 to113 around the gate contacts. This process comprises applying current tothe gate contact of the substrate being immersed into an electrolyticsolution. In carrying out this step, preferably, the anodic oxide filmon the TFT (the TFT on the left hand side of the figure) in the regionof the peripheral circuit is provided as thin as possible to increasethe mobility, while providing a thick anodic oxide film on the portioncorresponding to the TFT (the reverse staggered TFT located on the righthand side of the figure) of the active matrix portion to thereby preventgate leak. The anodic oxidation films in this example were formed atthicknesses in the range of from 200 to 250 nm.

Impurities were introduced into the island-shaped silicon film of eachof the TFTs by ion doping process in a self-aligned manner using thegate contact portion (i.e., the gate contact and the surrounding anodicoxide film) as the mask. In carrying this process, phosphorus wasimplanted first over the entire surface using phosphine (PH₃) as thedoping gas, and then boron was implanted using diborane (B₂H₆) as thedoping gas while covering the island portion 105 alone with aphotoresist, so that boron may be introduced only into the islandportion 106. Phosphorus and boron in this step were introduced at a doseof from 2×10¹⁵ to 8×10¹⁵ cm⁻² and from 4×10¹⁵ to 10×10¹⁵ cm⁻²,respectively, so that the dose of boron may be higher than that ofphosphorus.

The resulting structure was then subjected to laser beam irradiationusing a KrF excimer laser emitting a light at a wavelength of 248 nm andbeing operated at a pulse width of 20 nsec as shown in FIG. 1(D), sothat the crystallinity of the portion may be recovered from the damageit had received by the introduction of impurities. The laser beam wasapplied at an energy density of from 200 to 400 mJ/cm², and preferably,from 250 to 300 mJ/cm². A further homogeneous crystal can be obtained byapplying an auxiliary heat by heating in the temperature range of from100 to 500° C.

Thus was obtained N-type regions 114 and 115, and P-type regions 116 and117. The sheet resistivity of the regions was found to be in the rangeof from 200 to 800 Ω/sq.

Then, a 300 nm thick silicon oxide film was deposited over the entiresurface by sputtering as an interlayer insulator 118. This silicon oxidefilm may be replaced by a silicon nitride film having deposited byplasma CVD. The film thus obtained functions as a mere interlayerinsulator in a peripheral circuit, but care must be taken in itsfabrication when it is brought into an active matrix portion, becausethen it functions as a gate insulator for TFTs.

An amorphous silicon layer 119 was then deposited on the gate contact110 of the active matrix portion at a thickness of from 20 to 50 nm, anda microcrystalline silicon layer which serves as the source/drain of thea-Si TFT was deposited by plasma CVD at a thickness of from 50 to 100nm. The resulting microcrystalline silicon film was patterned to obtainsource/drain 120 and 121.

Contact holes were then perforated on the source/drain of the TFTs ofthe peripheral circuit portion to establish aluminum connection 122,123, and 124. It can be seen in this case that an inverter circuit isformed by the NTFT and the PTFT in the left hand side. Furthermore, apixel electrode 125 was formed with a light-transmitting electricallyconductive material such as an ITO on the TFT in the active matrixportion. Finally, the resulting structure was annealed in hydrogen for 2hours at 350° C. to reduce the dangling bonds in the silicon film toobtain a peripheral circuit being integrated monolithically with theactive matrix circuit. In the present example, a reverse-staggered typeTFT was used as the a-Si TFT of the active matrix to prevent incidentlight to enter the channel portion, because the electric conductivity ofan a-Si easily changes upon irradiation of light. Needless to say, aplanar TFT can be applied as well if an effective countermeasure wouldbe taken to shield the TFT from the external light irradiation.

An illustrative example of the characteristics of a TFT assembled in aperipheral circuit having fabricated according to the present example isshown in FIG. 6. The TFT was obtained by irradiating KrF laser beam invacuum to a 20 nm thick protective layer having formed on aLPCVD-deposited 50 nm thick silicon film to crystallize the siliconfilm. The KrF laser was operated at an energy density of 250 mJ/cm², and10 shots was applied to the film. Then, after removing the protectivelayer, a 120 nm thick silicon oxide film was deposited thereon bysputtering to obtain a gate dielectric. After forming a gate contact, a206 nm thick anodic oxide film was formed by anodic oxidation to use asa mask in the subsequent ion implantation. The ion implantation wasconducted by bombarding the structure with phosphorus ions beingaccelerated at an energy of 65 keV and boron ions being accelerated at80 keV to establish the impurity regions in a self-aligned manner, andactivation was conducted thereafter by irradiating 10 shots of pulsedKrF laser beam at an energy density of 300 mJ/cm² in air.

FIGS. 6(A) and 6(B) each show the characteristics of an NTFT and a PTFT,respectively. The channel of the TFT is 3.5 μm in length and 15 μm inwidth. The field mobility was found to reach 60 cm²/Vs in NTFT and 30cm²/Vs in PTFT. Furthermore, an S value, which shows the steepness ofthe ON/OFF of a TFT, was obtained as 0.42 V/digit for an NTFT and 0.53V/digit for a PTFT. The threshold voltage of the NTFT was found to be3.9 V, and that of the PTFT was found to be −5.4 V. The ON/OFF ratio fora drain voltage of 1 V or −1 V was obtained as 8.7 digits for the NTFTand as 6.9 digits for the PTFT.

EXAMPLE 2

The present example provides an active matrix having formed on asoda-lime glass substrate. Because a soda-lime glass is rich in sodium,a silicon nitride coating 202 was deposited by plasma CVD at a thicknessof from 5 to 50 nm, preferably, at a thickness of from 5 to 20 nm, overthe entire surface of a soda-lime glass substrate 201 having a thicknessof 1.1 mm and an area of 300×400 mm. The silicon nitride coating aboveprevents sodium from diffusing into the TFT from the soda-glasssubstrate. This technology of providing a blocking layer on a substrateby coating the substrate with a silicon nitride or an aluminum oxidefilm is disclosed in Japanese Patent Application Nos. Hei-3-238710 andHei-3-238714 filed by the present inventors. In addition, the coating202 may otherwise be an aluminum nitride film.

Then, after forming a silicon oxide film as a base oxide film 203, asilicon film 204 was deposited by plasma CVD or LPCVD process at athickness of from 30 to 150 nm, preferably from 30 to 50 nm, and asilicon oxide film was deposited thereon as a protective layer 205. Theresulting structure was then subjected to the irradiation of a KrFexcimer laser as shown in FIG. 2(A) to improve the crystallinity of thesilicon film 204. Laser beam was applied at an energy density of from150 to 200 mJ/cm²; i.e., at a value slightly lower than that of thelaser beam used in Example 1. Furthermore, only 10 shots were applied.Accordingly, the crystallinity of the silicon film thus deposited wasfound to be closer to an amorphous state than the one obtained inExample 1. In fact, the hole mobility of the silicon film in this casewas found to yield, more specifically, a value in the range of from 3 to10 cm²/Vs, a value lower than that obtained in the silicon film ofExample 1.

Subsequently, the protective layer was removed to expose the siliconlayer, and the exposed surface was patterned into an island-like shapedregion 206 to establish thereon a gate oxide film 207 at a thickness offrom 50 to 300 nm, preferably, at a thickness of from 70 to 150 nm bysputtering. An aluminum film was deposited and patterned thereafter inthe same manner as in Example 1 to obtain a gate contact 208, and thegate contact was surrounded with an anodic oxide 209. The resultingstructure is shown in FIG. 2(B).

Boron was then introduced as a P-type impurity into the silicon layer ina self-aligned manner by ion doping, to thereby form source/drain 210and 211 of the TFT. Subsequently, as illustrated in FIG. 2(C), theresulting structure was subjected to laser beam irradiation using a KrFexcimer laser, so that the crystallinity of the portion may be recoveredfrom the damage it had received by the introduction of impurities. Thelaser beam was applied at a rather high energy density of from 250 to300 mJ/cm². Accordingly, a sheet resistivity in the range of from 400 to800 Ω/sq., a value well comparable to the one obtained in Example 1 wasobtained for the source/drain of the TFT.

Thus was obtained a TFT comprising an active layer having small fieldmobility, but best suited for use in an active matrix. Morespecifically, the TFT obtained in this example has a high ON resistance,however, the OFF resistance thereof is still sufficiently higher thanthe ON resistance. Accordingly, an additional capacitance which wasconventionally necessary is no longer required. In particular, thesource of a leak current in an N-channel MOS, i.e., the mobile ions suchas sodium, casts no problem in a P-channel type device as referred inthe present example.

The process according to the present example can be conducted at lowtemperatures with 350° C. being the maximum limit. The highest allowabletemperature is attained at the fabrication of a silicon nitride film ora silicon oxide film. If the temperature were to be elevated as toexceed the maximum limit, the soda-lime glass would soften. In a processof such a low temperature, the defects in the gate oxide film sometimescauses problems. In the case of Example 1, the gate oxide film wasannealed at a temperature lower than 450° C. because the substrate had arelatively high heat resistance to allow thermal annealing at such ahigh temperature. In the present Example using soda-lime glasssubstrate, however, such thermal annealing cannot be applied.Consequently, a large number of fixed charges, which are principallypositive ones, would remain inside the gate oxide film. It follows thatthe resulting structure is not applicable to N-channel type MOS due tothe excessively large leak current which generates by the presence ofthose fixed charges. In a P-channel type MOS, however, though the fixedcharge certainly affects the threshold voltage, the leak current can besuppressed low so that the essential characteristic for an active matrixoperation can be achieved. Furthermore, the source/drain were annealedwith a high energy density laser beam to yield a low sheet resistance.This leads to the suppression of signal delay.

An interlayer insulator 212 was formed with polyimide thereafter. Thisstep was followed by the formation of pixel electrode 213 using ITO. Acontact hole was then established to form aluminum contacts 214 and 215in the source/drain regions of the TFT. One 215 of the thus formedcontacts was connected to the ITO. Finally, the hydrogenation of siliconwas completed by annealing the resulting structure in hydrogen at 300°C. for 2 hours.

Four active matrices were formed on the resulting single substrate, andthe entire structure was cut into four pieces to obtain four activematrix panels. The thus obtained active matrix has no peripheralcircuit, and it can be driven only after connecting a driver IC theretoby a TAB process and the like. However, since a low cost soda-lime glasssubstrate is used in the place of an alkali-free glass substrateconventionally employed in an a-Si TFT AMLCD, the total cost is wellcomparable to those of the conventional panels. In particular, the panelaccording to the present Example was found best suited for large areafine displays. The active matrix thus obtained is shown schematically inFIG. 11. The active matrix 952 is connected to a peripheral circuit 951comprising a driver TFT and a shift resister. A pixel 953 of the activematrix comprises a TFT 956, a liquid crystal layer 954, and an auxiliarycapacitance 955.

An a-Si TFT, for instance, has a mobility in the range of about 0.5 to1.0 cm²/Vs, and was not applicable to large scale matrices which wouldexceed 1,000 lines. In contrast, the TFT according to the presentexample has a mobility as high as 3 to 10 times that of the conventionalTFTs that it can be applied to such large scale matrices without anyproblem. Moreover, it would satisfactorily respond to analog-likegradation displays.

Furthermore, since the gate lines and the data lines are both made fromaluminum, signal delay and attenuation can be considerably reduced evenin a large display exceeding 20 inches in diagonal.

EXAMPLE 3

The present example provides a high contrast LCD taking advantage ofboth the diode characteristics and the memory function of aferroelectric polymer, using a process having the fabrication cost beingreduced by integrating the peripheral circuit monolithically on a singlesubstrate. The LCDs having similar structures can be found disclosed in,for example, Japanese Patent Application No. Sho-61-1152.

This type of LCD allows a semi-static operation. Accordingly, a displayof an extremely high contrast can be obtained despite a TN liquidcrystal is operated in a direct multiplexing drive. Moreover, incontrast to MIM type non-linear elements, no problems are encountered inthe fabrication process. The principle of its operation is illustratedin FIG. 4.

In general, the E (electric field)-(electric flux density)characteristics of a ferroelectric exhibits a hysteresis curve asillustrated in FIG. 4(A). That is, the constant polarization which isformed in a ferroelectric material under a certain external electricfield happens to be reversed upon application of an electric fieldexceeding a certain value. If an electric circuit is considered, thissignifies charge transfer, and hence, current is generated therein. Forinstance, a serial connected circuit comprising a capacitor (FE) havingincorporated a ferroelectric between the electrodes and a capacitor (LC,having a capacitance of C) having incorporated a material such as aliquid crystal between the electrodes may be considered. In practicaluse, a relatively high resistance R is generally parallel connected witha ferroelectric capacitor. Accordingly, a practical circuit results in aconstruction as illustrated in FIG. 4(C). It should be noted, however,that FE not only functions as a simple capacitor, but also as anon-linear resistor. The change of the current in the resulting circuitupon applying an alternating current to the circuit can be obtained asillustrated in FIG. 4(B), exhibiting non-linear characteristics to yielda hysteresis curve.

When a voltage of −V₀ or 0 is applied to one of the facing electrodeswhile applying a voltage of 0 or +V₀ to the other, the voltage of thecell results in one of ±2V₀, ±V₀, and 0. If the voltage results ineither +2V₀ or −2V₀, as illustrated in FIG. 4(B), the resistance of FEdrops abruptly during its transition, and this signifies that asufficient amount of charge is supplied to LC. If the voltage transitionthen occurs to yield any of the values +V₀, −V₀ and 0, the resistance ofFE would not drop considerably this time. Consequently, the leak currentfrom the parallel connected resistance R should be taken intoconsideration in this process. The discharge of the LC occurs by theleak current. It can be seen therefore that the stage in ±2V₀corresponds to a selection stage, and that the other stages correspondto a non-selection stage.

Referring to FIG. 4(B), the straight line passing through the origin anddrawn with a dot-line curve represents the leak current due to R, andthe relation between R and C is particularly important in using thedevice as an LCD. Though not discussed in detail here, only a lowcontrast would result if the time constant τ=RC should fall considerablyshorter than the period of a frame because this signifies that thecontribution of FE is very small. On the other hand, if τ should beexceedingly longer than the period of a frame, the result would be anannoying display full of after images. Accordingly, τ should be set asclose as possible to the period of a frame.

A cell is schematically shown in FIG. 5. This cell is so constructedthat a liquid crystal material 512 is sandwiched between two substrates501 and 502, in the same way as an ordinary LCD. To make the thicknessof the cell uniform, spacers 511 are interposed between the substrates.Usable liquid crystal materials include twisted-nematic liquid crystals,super twisted-nematic liquid crystals, untwisted nematic liquid crystalsusing birefringence, ferroelectric liquid crystals, and dispersion typeliquid crystals (PDLCs) comprising a polymer in which a liquid crystalsuch as a nematic liquid crystal or cholesteric liquid crystal isdispersed.

In the same manner as generally adopted simple matrix liquid crystals,transparent stripe-shaped electrodes 505 and 506 made of ITO or anothersimilar material are arranged so as to intersect each other at rightangles. The difference with an ordinary simple matrix structure is thata transparent conductive coating of ITO or another similar materialtaking the form of islands is formed over one electrode 506 with aferroelectric polymer 507 therebetween. Orientation films 509 and 510are formed so as to cover these electrodes. This structure is describedin detail in Japanese Patent Application No. 1152/1986.

The LCD constructed in this way is driven by a TAB connection of ICs ina conventional manner. This configuration has some limitations. First,in the LCD of this system, the voltage applied to the liquid crystalassumes value 1 or 0. This voltage is kept applied throughoutsubstantially one whole frame to achieve high contrast, which is onefeature of this system. Accordingly, when an image is displayed atvarious gray levels, it is difficult to accomplish an analog gray-scaledisplay, which is usually done in TFT LCDs. Also, neither the pulsemodulation method nor the frame modulation method, which are employed inSTN LCDs, can be adopted. As a result, the LCD relies on atwo-dimensional gray scale. This greatly increases the number of pixels.

The above itself is not an intrinsic difficulty with this LCD. That isto say that a large-capacity matrix can be rather easily attained due tothe simple structure of this kind of LCD. In practice, however, wherethe density of connected terminals reaches 20 lines/mm, it is no longerpossible to cope using the TAB system. Furthermore, it is difficult tofabricate the LCD by the COG (chip-on-glass) method. Therefore, it hasbeen required that a peripheral driver circuit be formed monolithicallyon the same substrate.

For example, in order to achieve a two-dimensional gray scale with 64gray levels, 6 subsidiary pixels are needed per pixel. Hence, the numberof rows needed is two or three times as many as the number of rows in anormal matrix structure. If the present system is adopted in ahigh-definition screen in compliance with XGA standards, the number ofrows reaches 1500 to 3000. Even in the case of a 15-inches. diagonallarge-sized screen, a density of 10 to 15 lines/mm is needed. As thescreen is narrowed, a higher-density packing is required. Especially,where a projection-type display is built using both the present systemof LCD and a high-transmittance liquid crystal PDLC, the diagonal of thesubstrates is less than 5 inches.

At this time, high-speed IC operation is necessitated in addition tohigh-density packing. In this case, a circuit on an insulating substrateis less liable to loss than a circuit on a semiconductor substrate of asingle crystal and can operate at a higher speed. However, if the fieldmobility is less than 10 cm²/V·s as in Example 2, problems occur usingthat configuration. Therefore, it is required that the mobility be morethan 30 cm²/V·s, preferably more than 50 cm²/V·s.

For this reason, a low-temperature process using laser annealing orintense light similar to laser is desired. A process for fabricating aperipheral circuit described in FIG. 3 is described below. Corning 7059or other similar non-alkaline glass substrate is used as a substrate301. This substrate measures 300 mm by 400 mm. An oxide film 302 made ofsilicon oxide is formed as a base layer on the substrate. In addition, asilicon layer 303 and a protective layer 304 are formed. As shown inFIG. 3(A), the laminate is irradiated with laser radiation under thesame conditions as in Example 1.

Subsequently, the silicon layer is photolithographically patterned intoislands to form NTFT regions 305 and PTFT regions 306 and a gate oxidefilm 307 is fabricated from silicon oxide. As shown in FIG. 3(B),aluminum gate electrodes 308 and 309 are formed. Since the aluminum hasto withstand later laser irradiation, the aluminum gate electrodes areformed by electron-beam vaporization so as to impart high reflectivitythereon. Aluminum grains formed by sputtering are as large as about 1μm, and the surface of the aluminum coating is very rough. Therefore,when the aluminum coating is irradiated with laser radiation, thecoating is damaged severely. The aluminum film formed by electron-beamevaporation has a surface which is so flat that the existence of grainscannot be confirmed by an optical microscope. Observation with anelectron microscope reveals that the grain sizes are less than 200 nm.That is, the grain sizes must be smaller than the wavelength of thelaser radiation used.

Thereafter, N-type dopant or phosphorus ions is/are implanted intoregions 310 and 311. P-type dopant or boron ions is/are introduced intoregions 312 and 313. As shown in FIG. 3(C), a laser annealing process isperformed. Laser irradiation is carried out under the same conditions asin Examples 1 and 2. During this process, the aluminum gate electrodesare essentially undamaged.

Finally, as shown in FIG. 3(D), an interlayer insulator 314 isfabricated from silicon oxide and contact holes are formed therein.Aluminum interconnectors 315-317 are formed to interconnect the TFTs. Aperipheral circuit is thus completed. Then, a stripe-shaped ITO film(not shown), and pixel electrodes are formed. The substrate is dividedinto quarters, each measuring 150 mm by 200 mm. In this manner, foursubstrates are obtained. A ferroelectric polymer or the like isdeposited on two of the substrates by the method described in theabove-cited Japanese Patent Application No. 1152/1986, then twosubstrates are bonded together as shown in FIG. 5 to complete an LCD.

EXAMPLE 4

The present example is illustrated in FIG. 7, which lies in the use ofnovel laser-crystallized silicon TFTs (thin film transistor) in aperipheral circuit for a TFT liquid-crystal display. Unlike Example 1,TFTs (thin film transistor) in an active-matrix circuit region are madeof top-gate amorphous silicon, i.e., the gates are located on theopposite side of the substrate. In this case, the active layers of bothTFTs can be fabricated by the same manufacturing process but undersomewhat stricter conditions, because better laser-crystallizedcharacteristics and better amorphous silicon characteristics arerequired. The active-matrix circuit and the peripheral circuit areprovided on an insulating substrate.

First, an oxide film 702 forming a base layer is deposited by sputteringon a substrate 701 of Corning 7059 up to a thickness of 20 to 200 nm. Asubstantially amorphous silicon film of monosilane or disilane isdeposited on the oxide film 702 by plasma CVD to a thickness of 50 to150 nm. At this time, the substantially amorphous silicon film isrequired to function directly as amorphous silicon TFTs and to withstandlaser irradiation. We have discovered that if the substrate temperatureis set to 300-400° C. during fabrication of a substantially amorphoussilicon film, then the characteristics of this substantially amorphoussilicon film are improved. A protective silicon oxide film 705 having athickness of 10 to 50 nm was formed on this substantially amorphoussilicon film again by sputtering. Subsequently, the active-matrixcircuit region is coated with a photoresist 706, and only the peripheralcircuit is irradiated with laser radiation to heighten a crystallinityof the substantially amorphous silicon film of the peripheral circuit.

Under this condition, laser irradiation is performed as shown in FIG.7(A). The kind of laser used and the conditions are the same as inExample 1. At this time, however, the energy density of the laser ispreferably 200 to 250 mJ/cm² for the following reason. An excessiveamount of hydrogen is contained in the amorphous silicon film formed byplasma CVD and if it is irradiated with intense laser radiation, thehydrogen changes into a gaseous state, expands, and destroys the film.The silicon film is crystallized as described above and a crystallizedregion 704 is formed. On the other hand, those portions which are coatedwith the photoresist are not illuminated by laser radiation and,therefore, are kept in an amorphous state.

Subsequently, the silicon film is patterned into islands to form islandregion 707 for the peripheral circuit and island region 708 for theactive-matrix region, as shown in FIG. 7(B). Silicon oxide is sputteredon these island regions to form a gate-insulating film 709. Metal gateelectrodes 710, 711, and 712 coated with an anodic oxide films onsurfaces thereof are formed in the same way as in Example 1.

Then, as shown in FIG. 7(C), an N-type impurity is introduced intoregions 713 and 715 with the gate electrodes 710 and 712 as masks. AP-type impurity is implanted into a region 714 with the gate electrode711 as a mask. These regions are then irradiated with laser radiationwith the gate electrodes as masks to crystallize the implanted regionsunder the same conditions as in Example 1. A high crystalline silicon isobtained therein. Regions 716 and 717 are already crystallized at thestage of FIG. 7(A), but a region 718 is not yet crystallized in thisstep. That is, in the TFT (the TFT in the active-matrix region) at theright side of FIG. 7, the source and drain crystallized but the activeregion is still a substantially amorphous silicon semiconductor. Theactive regions 716 and 717 of the thin film transistors in theperipheral circuit provided around the active matrix circuit comprise acrystalline semiconductor.

Finally, a silicon oxide film is deposited as an interlayer insulator719 to a thickness of 400 to 1000 nm by TEOS plasma CVD. Then an ITOfilm 720 having a thickness of 100 to 300 nm is formed in theactive-matrix region. This ITO film is patterned to form pixelelectrodes. Contact holes are formed in the interlayer insulator. Metalwiring layers 721-724 are formed on the interlayer insulator. Thus, aTFT active-matrix liquid-crystal display is fabricated.

In this liquid-crystal display, the active regions of the thin-filmtransistors in the active-matrix circuit have lower crystallinity thanthe active regions of the thin-film transistors in the peripheralcircuit. The active regions of the thin-film transistors in theactive-matrix circuit are a substantially amorphous silicon film whichexhibits a resistivity of 10⁹ Ω·cm or more in the dark.

In the present example, the TFTs forming the pixels are made ofamorphous silicon TFTs which show a high resistivity in an OFFcondition, in the same manner as in Example 1. However, the TFTs used inExample 1 are of the reverse-staggered type. In the present example, theTFTs are of the top-gate type. In Example 1, the step for fabricatingTFTs of a peripheral circuit and the step for fabricating the TFTs ofthe active-matrix circuit are different except for the process forfabricating the gate electrodes. In consequence, the number of steps isincreased. In the present example, the TFTs of the peripheral circuitand the TFTs of the active-matrix circuit are built at the same time.Hence, the number of manufacturing steps can be reduced.

A silicon film suitable as an amorphous silicon TFT is required tocontain a large amount of hydrogen. However, the hydrogen content mustbe reduced as much as possible to crystallize the TFT by laserirradiation. Since these two requirements are in conflict with eachother, a silicon film satisfying both conditions to a considerableextent must be formed. For example, where plasma CVD is used, if asilicon film is formed by the use of a high-energy plasma such as ECRplasma or microwave plasma, numerous crystallized clusters are containedin the film. This is ideal for the purpose of the present example.However, it presents the problem that resistivity in the OFF conditionis somewhat low.

EXAMPLE 5

The present example is illustrated in FIG. 8. In Examples 1-4, the TFTregions are separated to electrically isolate them from each other. Inthe present example, a silicon film is formed over the whole surface andselectively crystallized. Also, a thick insulating film is employed toisolate the TFTs from each other.

First, a silicon oxide film 802 forming a base layer is deposited on aninsulating substrate 801. A substantially amorphous silicon film havinga thickness of 50 to 150 nm or a silicon film having low crystallinitycomparable to the crystallinity of the substantially amorphous siliconfilm is formed on the silicon oxide film 802. In the present example, itis necessary that the substantially amorphous silicon film sufficientlywithstand laser irradiation and exhibit high resistivity. Therefore, thesubstantially amorphous silicon film is fabricated under the sameconditions as in Example 4. Then, a silicon oxide film (an insulatingfilm) having a thickness of 10 to 500 nm, preferably 10 to 50 nm, isformed over the entire surface of the substantially amorphous siliconfilm by plasma CVD. The silicon oxide film (insulating film) isselectively etched to obtain a region having removed therefrom thesilicon oxide film (insulating film) or having thinned therein thesilicon oxide film (insulating film). Thick silicon oxide film regions805 and thin silicon oxide film regions 806 are then formed. At thistime, if isotropic etching techniques are used, smoothly sloping stepsare formed as shown in FIG. 8(A). Hence, breakage of the metal wiringlayer which would otherwise be caused by steep steps can be prevented.

Under this condition, the laminate was lightly doped with boron ions andirradiated with laser light to crystallize the silicon film. As aresult, as shown in FIG. 8(A), the substantially amorphous silicon layerwas crystallized to heighten a crystallinity of portions 804 of thesilicon film provided under said region having removed therefrom thesilicon oxide film or having thinned therein the silicon oxide film.Other regions 803 were maintained in an amorphous state. The regions 804take a substantially intrinsic or weak P-type because of the borondoping.

This step may be carried out by a method as illustrated in FIG. 8(E). Inparticular, after forming a silicon oxide film, a coating which has athickness of 20 to 500 nm and is made of a material reflecting laserlight such as aluminum, titanium, chromium, or the like or a materialwhich does not transmit laser light is formed on the silicon oxide film.This coating is patterned photolithographically. Using this coating 819as a mask, the silicon oxide film is isotropically etched. Thus, thickregions 817 and thin regions 818 are formed in the silicon oxide layer.The laminate is then irradiated with laser radiation while leaving themask 819 behind, to selectively crystallize the amorphous silicon film.In this manner, crystallized regions 816 and amorphous silicon regions815 are formed.

As shown in FIG. 8(B), a gate oxide film 807, or silicon oxide, is thenformed to produce metal gate electrodes 808 provided with an anodicoxide. Since a wet etching method is used to etch the metal gates, theside surfaces of the gate electrodes taper. This shape is effective inpreventing the conductive wiring layer from breaking at theintersections.

As shown in FIG. 8(C), N-type regions 809 and P-type regions 810 arethen formed by selectively implanting at least one impurity into thesilicon film with the gate electrode and the thick silicon oxide filmregions 805 as masks by ion doping. These regions are irradiated withlaser radiation to activate them. Thereafter, as shown in FIG. 8(D), aninterlayer insulator 811 is deposited, and contact holes are formedtherein. Metal wiring layers 812-814 are formed to complete the circuit.In the present example, a large amount of opaque amorphous silicon isleft on the substrate and therefore this structure cannot be used as theactive-matrix region of an LCD, for example. However, it can be used asa peripheral circuit region or as a driver circuit for an image sensor.In the present example, if a relatively thick (in excess of 100 nm)active layer is needed, steps for isolating elements are low.Accordingly, the possibility of breakage of the conductive wiring layerscan be greatly reduced. This advantage is especially conspicuous inhigh-density integrated circuits.

EXAMPLE 6

The present example is illustrated in FIG. 9. Similarly to Example 5, asilicon layer is formed over the whole surface and selectivelycrystallized to isolate elements from each other. Breakage of wiringlayers can be prevented more effectively because an uneven oxide film asused in Example 5 is not used.

First, a silicon oxide film 902 forming a base layer is deposited on aninsulating substrate 901. A substantially amorphous silicon film havinga thickness of 50 to 150 nm or a silicon film having low crystallinitycomparable to that of the substantially amorphous silicon film is formedon the silicon oxide film 902. In the present example, it is alsonecessary that the substantially amorphous silicon film sufficientlywithstand laser irradiation and exhibit a high resistivity. Therefore,the substantially amorphous silicon film is fabricated under the sameconditions as in Example 4. Then, a protective film 905 of silicon oxidehaving a thickness of 20 to 100 nm is formed on the substantiallyamorphous silicon film. This silicon oxide film 905 can be left behindand subsequently form gate-insulating films for TFTs. As mentionedpreviously, it is to be noted that these TFTs have low mobility. Then, acoating having a thickness of 20 to 500 nm and made of a laserlight-reflective material such as aluminum, titanium, chromium or thelike, or a material which does not transmit laser light is formed on thesilicon oxide film. This coating is photolithographically patterned. Asshown in FIG. 9(A), using this coating 906 as a mask, a laser light isirradiated to the substantially amorphous silicon film to selectivelycrystallize the amorphous silicon layer. Thus, crystallized regions(crystalline semiconductor regions) 904 and substantially amorphoussilicon semiconductor regions 903 are formed.

Then, as shown in FIG. 9(B), metal gate electrodes 907 and 908 havinganodic oxide on surfaces of the gate electrodes are formed on the newlyformed gate-insulating film over the crystalline semiconductor regions904. Since a wet etching method is used to etch the metal gates, theslide surfaces of the gate electrodes taper. This shape is effective inpreventing the wiring layers from breaking at the intersections.Additionally, a photoresist 909 is applied and patterned to expose onlyN-channel TFTs.

Using the photoresist and the gate electrode as masks, an N-typeimpurity is implanted into the silicon film. In this condition, thelaminate is irradiated with laser radiation to activate these implantedregions 912. At this time, the amorphous silicon would be crystallizedunless the photoresist remains in regions other than the implantedregions. Where a relatively thick oxide film cannot be used to isolateelements as in the present example, leakage between the elements wouldundesirably result.

Similarly, with respect to P-channel TFTs, a photoresist 910 is applied.A P-type impurity is implanted while exposing only the P-channel TFTs,to form P-type doped regions 913. Then, as shown in FIG. 9(C), thelaminate is irradiated with laser radiation while leaving thephotoresist behind, to activate the already doped P-type regions 913. Inthe steps described thus far, the laser light does not impinge onregions 914 located between the N-type doped regions 912 and the P-typedoped regions 912. Hence, the intervening regions 914 remained in asubstantially amorphous silicon state. Accordingly, if wiring layers areformed on the overlying insulating coating 905 which is also agate-insulating film, and if an inverted layer is formed by the wiringlayers, the leakage current is infinitesimal, because the field mobilityof the amorphous silicon is very small and the resistivity is very high.In practice, no problems occur.

Then, as shown in FIG. 9(D), an interlayer insulator 915 is deposited,and contact holes formed in this insulator. Metal wiring layers 916-918are formed, thus completing the circuit. In the present example, a largeamount of opaque amorphous silicon is left on the substrate in the sameway as in Example 5 and therefore this configuration cannot be used forthe active-matrix region of an LCD, for example, but can be employed asa peripheral circuit region or as a circuit for driving an image sensor.In the present example, almost no steps exist between the gateelectrodes, unlike Example 5. Consequently, the possibility of breakageof the wiring layers can be reduced dramatically. This advantage isespecially conspicuous in high-density integrated circuits.

FIG. 9(E) shows another cross section of a TFT circuit fabricated in thepresent example. This is a cross section taken along the phantom lineA-B of FIG. 9(D) through an N-channel TFT. As can be seen from thisfigure, crystallized doped regions 912, 913′ and an interveningelement-isolating region (a separation semiconductor) 914 are on thesame plane and, therefore, the gate electrode 907 is flat. A wiringlayer 917′ which is in contact with the doped region 913′ and with agate electrode 907 has steps only in the locations of the contact holesand in the location of the interlayer insulator. Neither steps of islandsemiconductor regions as in Example 1 nor steps of thick insulating filmfor isolation of elements as in Example 5 exist. This is advantageousfor manufacturing integrated circuits at a higher density with a highproduction yield. In the device shown in FIG. 9(D), the transistors areseparated from each other by the separation semiconductor 914 providedbetween the crystalline semiconductor regions 904.

EXAMPLE 7

An example in which an active-matrix circuit is formed on a soda-limeglass substrate is given below. A soda-lime glass substrate having athickness of 1.1 mm and measuring 300 mm by 400 mm is used as asubstrate 201. A SiO₂ film 216 is formed on the substrate 201, as shownin FIG. 10(A). Then, a film 202 of AlN, SiN, or Al₂O₃ is formed over thewhole surface of the substrate as shown in FIG. 10(A). Thereafter, stepsare effected in the same way as in Example 2 to complete anactive-matrix circuit. That is, after forming an oxide film 203, orsilicon oxide, forming a base layer, a silicon film 204 having athickness of 30 to 150 nm, preferably 30 to 50 nm, is formed by LPCVD orplasma CVD. Then, a protective layer 205 of silicon oxide is formed.

As shown in FIG. 10(A), the laminate is irradiated with KrF laserradiation to improve the crystallinity of the silicon film 204. At thistime, the energy density of the laser radiation is set to 150 to 200mJ/cm², which is slightly lower than the energy density used inExample 1. The number of shots of laser radiation is 10. Thecrystallinity of the resulting silicon film is closer to an amorphousstate than in Example 1. In practice, the field mobility of positiveholes in the silicon film obtained under this condition is 3 to 10cm²/V·s, which is lower than the field mobility obtained in Example 1.

Then, the protective film is removed, and the silicon film is patternedinto island regions 206. A gate oxide film 207 having a thickness of 50to 300 nm, preferably 70 to 150 nm, is formed by sputtering. In the sameway as in Example 1, aluminum gate electrodes 208 are formed. These gateelectrodes 208 are coated with an anodic oxide 209, as shown in FIG.10(B).

Boron ions are then implanted as P-type dopant ions into the siliconlayer by self-alignment techniques to form the source/drain 210 and 211of each TFT. As shown in FIG. 10(C), the laminate is irradiated with KrFlaser radiation to improve the crystallinity of the silicon film, whichdeteriorates due to ion doping. At this time, the energy density of thelaser radiation is set to a comparatively high value of 250 to 300mJ/cm². Therefore, the sheet resistance of the sources/drains of theseTFTs is 400 to 800 Ω/cm², which is similar to the sheet resistanceobtained in Example 1.

Although the field mobility of the active layer is small, this smallmobility is advantageous when it is used as an active-matrix TFT. Inparticular, the ON resistivity is high but the OFF resistivity is higherstill. This makes it unnecessary to provide an auxiliary capacitance asin the prior art techniques. Especially, moving ions such as sodium ionscauses a leakage current from an N-channel MOS. In the present example,problems do not occur because it is of the P-channel type.

In the present example, the highest process temperature available is350° C. during fabrication of the silicon nitride film or silicon oxidefilm. The soda-lime glass softens at higher temperatures. Where such alow-temperature process is needed, defects in the gate oxide film poseproblems. In Example 1, the heatproofness of the substrate is relativelygood and so the gate oxide film can be annealed up to a temperature of450° C. This is impossible to achieve in the case of soda-lime glasssubstrates. The result is that numerous fixed charges are left in thegate oxide film. In this case, the fixed charges are primarily positivecharges. Therefore, an N-channel MOS produces a large amount of leakagebetween the source and drain under the influence of the fixed chargesand so the N-channel MOS cannot be employed in practice. However, in aP-channel MOS, fixed charges affect the threshold voltage but thelow-leakage property essential for the operation of an active-matrixcircuit is maintained. Since the sources/drains are annealed by ahigh-energy laser beam, sheet resistance is low, and the delay ofsignals is suppressed.

Thereafter, an interlayer insulator 212 is fabricated from polyimide.Pixel electrodes 213 are formed from ITO, and contact holes are formed.Electrodes 214 and 215 of aluminum are formed on the source/drainregions of TFTs. One electrode 215 is also connected with the ITOelectrodes. Finally, the laminate is annealed within a hydrogenatmosphere at 300° C. for 2 hours, thus completing hydrogenation of thesilicon.

Four active-matrix circuits are formed on one substrate fabricated inthis way. The substrate is divided into four active-matrix panels. Inthe present example, the active-matrix circuits have no peripheralcircuits. Therefore, driving ICs must be connected with peripheralcircuits by TAB or the like. Since the substrate is made of soda-limeglass which is cheaper than the non-alkaline glass substrate used in theprior art amorphous silicon TFT-AMLCD, the substrate in the presentexample is sufficiently profitable. Especially, the panel fabricated inthe present example is suited for a large-sized, high-definition panel.The obtained active matrix is schematically shown in FIG. 11, where theactive matrix is indicated by 952. A peripheral circuit 951 has a driverTFT and a shift register. One pixel of the active matrix is indicated byreference numeral 953. Also shown are a TFT 956 of the active matrix, aliquid crystal layer 954, and an auxiliary capacitance 955.

In the prior art amorphous silicon TFT, the mobility is on the order of0.5 to 1.0 cm²/V·s. Hence, it has been impossible to apply this TFT to alarge-sized matrix having more than 1000 rows. In the present example,the mobility is 3 to 10 times the mobility of the amorphous silicon andtherefore problems do not occur. In addition, TFTs in the presentexample can sufficiently respond to analog gray-scale representation.Further, the gate lines and data lines are made of aluminum. In alarge-sized screen having a diagonal exceeding 20 inches, delay andattenuation of signals can be greatly reduced.

EXAMPLE 8

An example of the fabrication of a TFT according to the presentinvention is illustrated in FIG. 12. First, a substrate 1101 of Corning7059 measuring 300 mm by 300 mm or 100 mm by 100 mm is prepared. Analuminum nitride film 1102 having a thickness of 1000 to 2000 Å isdeposited by reactive sputtering techniques. Using the aluminum as atarget, a sputtering process is performed in an atmosphere of nitrogenand argon. Where the ratio of the nitrogen is 20% or more, a coatinghaving good thermal conductivity is derived. Where the pressure at thetime of sputtering is 1×10⁻⁴ to 1×10⁻² torr, favorable results can beobtained. The deposition rate is 20 to 200 Å/min. During deposition, thesubstrate temperature can be increased to 100-500° C.

The aluminum nitride film 1102 is formed on both faces of the substrateto confine foreign elements such as sodium, either contained in thesubstrate or adhered to the surface after shipment, for preventingdeterioration in the characteristics of the TFTs. The aluminum nitridefilm 1102 also serves to reinforce the surface of the substrate, forpreventing the surface from being scratched. Especially, where TFTs areused in an active-matrix liquid-crystal display, the surface having noTFTs is exposed to the external environment and easily scratched. Ifscratches are formed, they reflect light irregularly, thereby darkeningthe screen. After the formation of the aluminum nitride film, an oxidefilm 1103 acting as a base layer and having a thickness of 1000 to 3000Å is formed on the surfaces on which TFTs are to be formed. To form thisoxide film, sputtering may be performed in an oxygen atmosphere.Alternatively, TEOS may be decomposed and deposited by plasma CVD in anambient of oxygen, and the resulting film may be annealed at 450 to 650°C.

Then, an amorphous silicon film is deposited to a thickness of 300 to1500 Å, preferably 500 to 1000 Å, by plasma CVD or LPCVD. This film isphotolithographically patterned into island silicon regions 1104. Asilicon oxide film having a thickness of 200 to 1500 Å, preferably 500to 1000 Å, is then formed. This silicon oxide film serves also as agate-insulating film. Therefore, sufficient care must be paid infabricating this film. In the present example, the film is fabricatedfrom TEOS. TEOS is decomposed and deposited together with oxygen at asubstrate temperature of 150 to 400° C., preferably 200 to 250° C., byRF plasma CVD. The ratio of the pressures of TEOS and oxygen is 1:1 to1:3. The pressure is 0.05 to 0.5 torr. The RF power is 100 to 250 W.Alternatively, the film can be fabricated from TEOS together with ozonegas by low-pressure CVD or atmospheric pressure CVD at a substratetemperature of 150 to 400° C., preferably 200 to 250° C. After theformation of the film, the laminate is annealed at 300-500° C. for 30 to60 minutes in an atmosphere of oxygen or ozone.

Then, as shown in FIG. 12(A), the laminate is irradiated with KrFexcimer laser radiation having a wavelength of 248 nm and a pulse widthof 20 nsec to crystallize the silicon region 1104. The energy density ofthe laser radiation is 200 to 400 mJ/cm², preferably 250 to 300 mJ/cm².During laser irradiation, the substrate is heated to 300 to 500° C. Thecrystallinity of the silicon film 1104 formed in this way has beenexamined by Raman spectroscopy, and a relatively broad peak was observedin the vicinity of a 515 cm⁻¹ different from the peak (521 cm⁻¹) of thesingle crystal of silicon. Then, the laminate is annealed at 350° C. for2 hours in an atmosphere of hydrogen.

Subsequently, an aluminum film having a thickness of 2000 Å to 5 μm isformed by electron-beam evaporation and patterned photolithographicallyto form gate electrodes 1106. The aluminum can be doped with 0.5 to 2%silicon. The substrate is immersed in an ethylene glycol solution of1-3% tartaric acid having a pH of about 7. The substrate is anodizedwhile using a platinum plate as a cathode and this gate electrode ofaluminum as an anode. At the beginning of anodization, the appliedvoltage is increased up to 220 V with a constant current. This conditionis maintained for 1 hour and then the process is ended. In the presentexample, the appropriate rate at which the voltage is increased is 2 to5 V/min. under the constant-current state. In this way, an anodic oxide1107 having a thickness of 2000 Å is formed (FIG. 12(B)).

Subsequently, impurity ions, or phosphorus ions, are implanted into theisland silicon regions of TFTs by a self-aligning ion doping process(also known as a plasma doping process) while using the gate electrodesas a mask. Phosphine (PH₃) is used as the doping gas. The dose is 2 to8×10¹⁵ ions/cm².

Then, as shown in FIG. 12(C), the laminate is irradiated with KrFexcimer laser radiation having a wavelength of 248 nm and a pulse widthof 20 nsec to improve the crystallinity of the silicon film, whichdeteriorates due to the ion doping. At this time, the energy density ofthe laser radiation is 150 to 400 mJ/cm², preferably 200 to 250 mJ/cm².In this way, N-type phosphorus-doped regions 1108 and 1109 are formed.The sheet resistance of these regions is 200 to 800 Ω/cm².

Then, a silicon oxide film is deposited as an interlayer insulator 1110having a thickness of 3000 Å over the whole surface by plasma CVD usingboth TEOS and oxygen or by low-pressure or atmospheric-pressure CVDusing TEOS and ozone. The substrate temperature is 150 to 400° C.,preferably 200 to 300° C. After the formation of the film, this siliconoxide film is mechanically polished to flatten the surface. Furthermore,ITO is deposited by sputtering and patterned photolithographically toform pixel electrodes 1111 (FIG. 12(D)).

As shown in FIG. 12(E), contact holes are formed in the source/drainregion of each TFT. Wiring layers 1112 and 1113 of chromium or titaniumnitride are formed, the wiring layer 1113 being connected with the pixelelectrodes 1111. Finally, the laminate is annealed at 200 to 300° C. for0.1 to 2 hours in an ambient of hydrogen, thus completing hydrogenationof the silicon. In this manner, TFTs are completed. Numerous TFTsmanufactured at the same time are arranged in rows and columns to buildan active-matrix liquid-crystal display.

EXAMPLE 9

An example of fabrication of an TFT according to the invention isillustrated in FIG. 13. First, an aluminum nitride film 402 having athickness of 1000 to 2000 Å is deposited on a substrate 401 of Corning7059 by a reactive sputtering method. Using the aluminum as a target,sputtering is effected in an atmosphere of nitrogen and argon. Where thenitrogen accounts for more than 20%, a coating having good thermalconductivity is derived. During sputtering, a pressure of 1×10⁻⁴ to1×10⁻² torr produces favorable results. The deposition rate is 20 to 200Å/min. During deposition, the substrate temperature can be increased to100 to 500° C.

Then, a silicon oxide film having a thickness of 1000 to 3000 Å isformed as an oxide film 403 forming a base layer. To form this oxidefilm, sputtering may be carried out in an oxygen atmosphere.Alternatively, TEOS may be decomposed and deposited by plasma CVD in anambient of oxygen, and the resulting film may be annealed at 450 to 650°C.

Thereafter, an amorphous silicon film having a thickness of 1000 to 3000Å, preferably 1000 to 1500 Å, is deposited by plasma CVD or LPCVD. Thelaminate is annealed at 600° C. for 48 hours in a nitrogen atmosphere.The obtained crystalline silicon film is patterned photolithographicallyinto island silicon regions 404. Silicon oxide is deposited as agate-insulating film 407 having a thickness of 200 to 1500 Å, preferably500 to 1000 Å.

An aluminum film having a thickness of 2000 Å to 5 μm is formed byelectron-beam evaporation and photolithographically patterned. Thelaminate is anodized under the same conditions as in Example 8 to formgate electrodes 409 and wiring layers 408. Then, dopant ions, orphosphorus ions, are implanted into the island silicon regions of TFTsby a self-aligning ion doping process (also known as a plasma dopingprocess) while using the gate electrodes as a mask. Phosphine (PH₃) isused as doping gas. The dose is 2 to 8×10¹⁵ ions/cm².

The laminate is irradiated with KrF excimer laser radiation having awavelength of 248 nm and a pulse width of 20 nsec to improve thecrystallinity of the silicon film, which deteriorates due to ion doping.The energy density of the laser radiation is 150 to 400 mJ/cm²,preferably 200 to 250 mJ/cm². In this way, N-type doped regions 405 and406 are formed. The sheet resistance of these regions is 200 to 800Ω/cm² (FIG. 13(A)).

Then, silicon oxide is deposited as an interlayer insulator 410 having athickness of 3000 Å over the entire surface by plasma CVD, LPCVD, oratmospheric pressure CVD. A photoresist 411 is selectively applied. Itis better to apply this photoresist at the intersections of wiringlayers or at locations where contacts are attached to the wiring layers(FIG. 13(B)).

As shown in FIG. 13(C), using the photoresist 411 as a mask, theinterlayer insulator 410, the gate insulator 407, and the base film 403of silicon oxide are etched. Although the base film is etched away, thesubstrate is not etched away because the aluminum nitride film acts as astopper. In this manner, a flat surface is obtained (FIG. 13(C)).

A titanium film having a thickness of 2000 Å to 5 μm is formed as aconductive interconnect material. This titanium film is patternedphotolithographically to form wiring layers 412 and 413 connected withthe source and drain of a TFT. ITO is selectively formed to producepixel electrodes 414. Finally, the laminate processed in this way isannealed in a hydrogen at 350° C. for 30 minutes at 1 atm., thuscompleting hydrogenation of the laminate. In this way, one TFT iscompleted. Numerous TFTs manufactured at the same time were arranged inrows and columns to build an active-matrix liquid-crystal display.

EXAMPLE 10

An example of fabrication of a TFT according to the invention isillustrated in FIG. 14. First, an aluminum nitride film 602 having athickness of 2000 to 4000 Å is deposited on a substrate 601 of Corning7059 by a reactive sputtering method. Using the aluminum as a target,sputtering is effected in an atmosphere of nitrogen and argon. Where thenitrogen accounts for more than 20%, a coating having good thermalconductivity is derived. During sputtering, a pressure of 1×10⁻⁴ to1×10⁻² torr produces favorable results. The deposition rate is 20 to 200Å/min. During deposition, the substrate temperature can be raised to 100to 500° C.

Then, a silicon oxide film is deposited as an oxide film 603 having athickness of 1000 to 2000 Å and forming a base layer. To form this oxidefilm, sputtering may be carried out in an ambient of oxygen.Alternatively, TEOS may be decomposed and deposited by plasma CVD in anambient of oxygen, and the resulting film may be annealed at 450 to 650°C.

Subsequently, an amorphous silicon film having a thickness of 1000 to3000 Å, preferably 1000 to 1500 Å, is deposited by plasma CVD or LPCVD.The laminate is annealed at 600° C. for 48 hours in a nitrogenatmosphere. The obtained crystalline silicon film is patternedphotolithographically into island silicon regions 604. Silicon oxide isdeposited as a gate-insulating film 605 having a thickness of 200 to1500 Å, preferably 500 to 1000 Å.

An aluminum film having a thickness of 2000 Å to 5 μm is formed byelectron-beam evaporation and photolithographically patterned. Thelaminate is anodized under the same conditions as in Example 8 to formgate electrodes 606 and a wiring layer 607 (FIG. 14(A)).

Then, impurity ions, or phosphorus ions, are implanted into the islandsilicon regions of TFTs by a self-aligning ion doping process (alsoknown as a plasma doping process) while using the gate electrodes as amask. Phosphine (PH₃) is used as doping gas. The dose is 2 to 8×10¹⁵ions/cm² (FIG. 14(B)).

The silicon oxide film 603 acting as a base layer is etched. The etchingis terminated by the aluminum nitride film 602 acting as a stopper.Under this condition, the laminate is irradiated with KrF excimer laserradiation having a wavelength of 248 nm and a pulse width of 20 nsec toimprove the crystallinity of the silicon film, which deteriorates due toion doping. The energy density of the laser radiation is 100 to 400ml/cm², preferably 100 to 150 mJ/cm². Since a silicon oxide filmcontaining phosphorus or boron absorbs ultraviolet radiation, wherelaser annealing is conducted subsequent to through-doping as in Example8, intense laser light is needed. In the present example, however, ifthe silicon oxide film, or gate-insulating film, is removed afterdoping, less laser energy suffices. This can improve the throughput ofthe laser processing. In this way, N-type phosphorus-doped regions 608and 609 are formed. The sheet resistance of these regions is 200 to 800Ω/cm² (FIG. 14(C)).

Then, silicon oxide is deposited as an interlayer insulator 610 having athickness of 2000 to 3000 Å over the entire surface by plasma CVD,LPCVD, or atmospheric pressure CVD. An aluminum film having a thicknessof 2000 Å to 5 μm is formed as a wiring layer material. This aluminum isphotolithographically patterned to form wiring layers 611 and 612connected with the source and drain of a TFT. As shown, the wiring layer612 crosses the wiring layer 607 (FIG. 14(C)).

Finally, the laminate processed in this way is annealed in a hydrogen at350° C. for 30 minutes at 1 atm., thus completing hydrogenation of thelaminate. In this way, a TFT is completed. At the same time, the dopedregion is doped with boron to fabricate a P-channel TFT. A CMOS isfabricated. Typical field mobilities of the N-channel and P-channeltypes are 80 to 150 cm²/Vs and 40 to 100 cm²/Vs, respectively. We haveconfirmed that a shift register constructed from these TFTs operated at11 MHz when the drain voltage was 17 V.

Although a high voltage exceeding 20 V is applied to the gate and thedrain for a long time (about 96 hours), the characteristics deteriorateonly slightly. This is because heat generated locally in the TFTs isquickly dissipated, suppressing liberation of hydrogen atoms from theinterface with the semiconductor coating and from the interface with thegate-insulating film.

EXAMPLE 11

An active-matrix circuit is formed on a soda-lime glass' substrate 201by the steps illustrated in FIGS. 2(A)-2(E). An insulating film 202having a higher thermal conductivity than that of silicon oxide 203 isformed over the whole surface of the substrate 201. In the presentexample, the film 202 is made of aluminum nitride. The aluminum nitridefilm is formed by sputtering under the same conditions as in Example 8.

Then, a silicon oxide film 203 forming a base layer is formed in thesame way as in Example 2. Subsequently, a TFT illustrated in FIG. 2(E)is formed by performing steps similar to the steps of Example 2.

EXAMPLE 12

A laser-crystallized silicon TFT for forming a peripheral circuit and anamorphous silicon TFT for an active-matrix circuit are formed on asubstrate 701 of Corning 7059 by the steps illustrated in FIGS.15(A)-15(D). An insulating film 725 having a higher thermal conductivitythan the silicon oxide 702 is formed on the substrate 701. In thepresent example, the film 725 is made of transparent aluminum nitride.This aluminum nitride film 725 is formed by sputtering under the sameconditions as in Example 8. Subsequently, a silicon oxide film 702forming a base layer is deposited to a thickness of 20 to 200 nm.Thereafter, steps similar to the steps of Example 4 were carried out toform a TFT (thin film transistor) on the silicon oxide film 702 as shownin FIG. 15(D).

EXAMPLE 13

A TFT is formed by the steps shown in FIGS. 16(A)-16(E). An insulatingfilm 925 having a higher thermal conductivity than silicon oxide 902 isformed on an insulating substrate 901. In the present example, the film925 is made of aluminum nitride. This aluminum nitride film 925 isformed by sputtering under the same conditions as in Example 8. Asilicon oxide film 902′ forming a base layer is then formed.Subsequently, steps similar to the steps of Example 6 are effected toform a TFT as shown in FIG. 16(E).

EXAMPLE 14

A TFT is formed by the steps illustrated in FIGS. 17(A)-17(E). Aninsulating film 825 having a higher thermal conductivity than siliconoxide 802 is formed on an insulating substrate 801. In the presentexample, the film 825 is made of aluminum nitride. This aluminum nitridefilm 825 is formed by sputtering under the same conditions as in Example8. A silicon oxide film 802 forming a base layer is then formed.Subsequently, steps similar to the steps of Example 5 are effected toform a TFT as shown in FIG. 17(D).

The present invention permits fabrication of a TFT which shows highreliability even if a voltage is applied for a long time. In this way,the present invention is industrially very advantageous. Especially,where TFTs are formed on a substrate having a large area and used as anactive-matrix circuit or as a driver circuit, great industrialadvantages can be attained.

In accordance with the present invention, TFTs can be manufactured atlow temperatures with a quite high production yield. Various LCDstructures can be produced according to the invention as described inthe above examples, because characteristics required by TFTs can be setat will in the present invention.

Although not described in the above examples, the invention can beapplied to a three-dimensional IC structure where a semiconductorcircuit is built on a single-crystal IC or the like. The above-describedexamples principally pertain to the use of the invention in variousLCDs. Obviously, the invention may also be utilized in other circuitswhich are required to be formed on an insulating substrate such as animage sensor.

1. A semiconductor device comprising: a substrate having a front surfaceand a rear surface; a first insulating film comprising silicon oxideprovided over said front surface of said substrate; a second insulatingfilm comprising aluminum nitride and oxygen provided over said firstinsulating film; a third insulating film comprising oxide provided oversaid second insulating film; a transistor provided over said thirdinsulating film, said transistor having at least a channel formationregion, a source region, a drain region, a gate insulating film adjacentto said channel formation region, and a gate electrode adjacent to saidchannel formation region with said gate insulating film interposedtherebetween; an interlayer insulating film comprising a leveled uppersurface over said transistor; and a pixel electrode over said interlayerinsulating film, wherein crystallinity of said source region or thedrain region is higher than crystallinity of said channel region.
 2. Thedevice of claim 1 wherein said substrate is a glass substrate.
 3. Asemiconductor device comprising: a substrate comprising a front surfaceand a rear surface; a first insulating film comprising silicon oxideprovided over said front surface of said substrate; a second insulatingfilm comprising aluminum nitride and oxygen provided over said firstinsulating film; a third insulating film comprising oxide provided oversaid second insulating film; a transistor provided over said thirdinsulating film, said transistor having at least a channel formationregion, a source region, a drain region, a gate insulating film adjacentto said channel formation region, and a gate electrode adjacent to saidchannel formation region with said gate insulating film interposedtherebetween; an insulating film over said transistor; and a pixelelectrode over said insulating film, wherein crystallinity of saidsource region or the drain region is higher than crystallinity of saidchannel region.
 4. The device of claim 3 wherein said substrate is aglass substrate.
 5. A semiconductor device comprising: a substrate; afirst insulating film comprising silicon oxide over said substrate; asecond insulating film comprising aluminum nitride and oxygen formedover said first insulating film; a third insulating film comprisingoxide formed over said second insulating film; and a transistor providedover said third insulating film, said transistor having at least achannel formation region, a source region, a drain region, a gateinsulating film adjacent to said channel formation region, and a gateelectrode adjacent to said channel formation region with said gateinsulating film interposed therebetween, wherein crystallinity of saidsource region or the drain region is higher than crystallinity of saidchannel region.
 6. The semiconductor device according to claim 5,wherein said semiconductor device is an active matrix display device. 7.The semiconductor device according to claim 5, wherein saidsemiconductor device comprises a pixel portion and a driver portion oversaid substrate.
 8. The semiconductor device according to claim 5,wherein said transistor comprises: a semiconductor film formed on saidthird insulating film, said semiconductor film comprising at least saidchannel formation region, said source region, and said drain region; agate insulating film formed on said semiconductor film; and a gateelectrode formed on said gate insulating film.